Add remote --cd forwarding for app-server sessions (#16700)

Addresses #16124

Problem: `codex --remote --cd <path>` canonicalized the path locally and
then omitted it from remote thread lifecycle requests, so remote-only
working directories failed or were ignored.

Solution: Keep remote startup on the local cwd, forward explicit `--cd`
values verbatim to `thread/start`, `thread/resume`, and `thread/fork`,
and cover the behavior with `codex-tui` tests.

Testing: I manually tested `--remote --cd` with both absolute and
relative paths and validated correct behavior.


---

Update based on code review feedback:

Problem: Remote `--cd` was forwarded to `thread/resume` and
`thread/fork`, but not to `thread/list` lookups, so `--resume --last`
and picker flows could select a session from the wrong cwd; relative cwd
filters also failed against stored absolute paths.

Solution: Apply explicit remote `--cd` to `thread/list` lookups for
`--last` and picker flows, normalize relative cwd filters on the
app-server before exact matching, and document/test the behavior.

codex-rs/tui/src/cli.rs

@@ -95,6 +95,7 @@ pub struct Cli {
     pub dangerously_bypass_approvals_and_sandbox: bool,
 
     /// Tell the agent to use the specified directory as its working root.
+    /// In remote mode, the path is forwarded to the server and resolved there.
     #[clap(long = "cd", short = 'C', value_name = "DIR")]
     pub cwd: Option<PathBuf>,